olympic bar length - 5, 6 or 7 foot

i'm thinking of buying an olympic weight set and a power rack.
i have a question hopefully one of you guys can help me with regarding the length of an olympic bar. i know the standard is 7'. however, on this site:http://www.fitness-superstore.co.uk/...8C61B5CE5.html

they have 5' and 6' models too. which should i get and whats the advantages of them? i know the obvious is that the 7' can handle more weight but i doubt i'll be pushing the 5' to the max anytime soon as i've only recently started lifting. i have a standard 5' bar i've been using which i'm quite comfortable with so maybe i should just get a 5' olympic bar too, no?